The present report describes our results on: a) Molecular control of GM-CSF production and b) Synergism between CSF and IL-1 in inducing differentiation in myeloid cells. a) Based on previous experiments we hypothesized that the production of GM-CSF in T cells is under post-transcriptional control. The present report describes our experimental results in supporting this hypothesis. Northern analysis was used to determine the steady-state levels of GM-CSF mRNA and nuclear transcription analysis (runoff) was performed to determine transcriptional activity. GM-CSF mRNA could be detected only in TPA-stimulated EL-4 thymoma cells but not in unstimulated cells. However, treatment of unstimulated cells with cycloheximide enabled detection of GM-CSF mRNA by Northern analysis. Furthermore, protecting GM-CSF RNA transcripts isolated from unstimulated cells with complementary 32P-labeled RNA and separation of the hybrids on CF11 cellulose columns revealed the presence of GM-CSF mRNA. Additional experiments showed transcriptional activity of the GM-CSF gene in unstimulated cells. Moreover, utilizing a drug that inhibits transcription initiation had no effect on production of GM-CSF in stimulated cells. All these data support a post-transcriptional control of GM-CSF production. b) CSF present in mouse lung conditioned medium (MLCM) induces differentiation in the myeloid leukemia cell line, M1, to mature macrophages. Optimal concentrations of MLCM (10%) induce a sequential expression of differentiation markers such as FcR, C3bR, induction of lysozyme and morphological changes typical of terminal myeloid differentiation. Suboptimal concentrations of MLCM (1.25-2.5%) fail to induce full differentiation. However, when IL-1 alpha is added to such suboptimal concentrations of MLCM, the combination of both cytokines is followed by a complete sequence of differentiation similar to that of optimal concentrations of MLCM. These data support the suggestion that IL- 1 alpha has an effect on hematopoietic cells, and is capable of cooperating with CSF in inducing differentiation in myeloid cells.